1. Field of the Invention
The present invention relates to an optical microscope and in particular to a three-dimensional (3D) microscope and methods of measuring a patterned substrate (PS) in 3D.
2. Description of the Related Art
High Brightness Light Emitting Diode (HBLED) has generated tremendous interest among research communities and various industries due to its reliability, long lifetime, and environmental benefits when compared to conventional light sources. Typically, conventional HBLEDs are manufactured on transparent substrates such as sapphire, silicon carbide, and other materials. To improve light extraction efficiency, manufacturers often roughen the substrate surface to form patterns so that a greater portion of light generated in the active layer can be emitted.
U.S. Pat. No. 6,657,236, entitled “Enhanced Light Extraction In LEDs Through The Use Of Internal And External Optical Element”, which issued to Thibeault on Dec. 2, 2003, and U.S. Pat. No. 7,384,809, entitled “Method Of Forming Three-Dimensional Features On Light Emitting Diodes For Improved Light Extraction”, which issued to Donofrio on Jun. 10, 2008, disclose methods of creating various repeating patterns on a silicon carbide substrate to enhance the light extraction efficiency of a HBLED. As described in U.S. Pat. No. 7,384,809, images from a secondary electron microscope (SEM) can be used to verify the shapes of these patterned substrates.
U.S. Pat. No. 7,683,386, entitled “Semiconductor Light Emitting Device With Protrusions To Improve External Efficiency And Crystal Growth”, which issued to Tanaka on Mar. 23, 2010, U.S. Pat. No. 7,745,245, entitled “Semiconductor Light Emitting Device”, which issued to Niki on Jun. 29, 2010, and U.S. Published Application 2008/0067916, entitled “Light Emitting Device Having A Patterned Substrate And The Method Thereof”, which was filed by Hsu on Jul. 30, 2007, teach various ways to generate repeating patterns on a sapphire substrate. In these references, SEM images are provided to confirm the quality of the patterned sapphire substrates.
U.S. Pat. No. 7,704,763, entitled “Highly Efficient Group-III Nitride Based Light Emitting Diodes Via Fabrication Of Features On An N-Face Surface”, which issued to Fuji on Apr. 27, 2010, discloses a method of manufacturing a HBLED on a sapphire substrate, then using laser lift-off to de-bond the substrate from the diode structure. At this point, an etch process can be used to create random pyramids on an N-face GaP surface to achieve a roughened surface. Again, images from a SEM can be used in monitoring formation of the random pyramid features.
As part of the manufacturing process development and process control, manufacturers need to measure the geometry of the pattern on the substrates. These measurements typically include the shape, height, size, pitch, and space of the pattern features. Although a conventional SEM can image various patterned features, it cannot measure height information. As a result, cross-sectional SEM (x-SEM) has become the standard metrology tool in the HBLED industry. However, x-SEM is a destructive method, which requires breaking of a HBLED prior to taking a measurement. In addition, x-SEM measurement has to be carried out in a vacuum environment and therefore is slow in throughput. Furthermore, an x-SEM system is expensive to buy and maintain.
Non-destructive, non-contact optical systems have been used in the semiconductor industry for years in measuring masks on transparent substrates. For example, U.S. Pat. No. 6,323,953, entitled “Method And Device For Measuring Features On A Transparent Substrate”, which issued to Blaesing-Bangert on Nov. 27, 2001, and U.S. Pat. No. 6,539,331, entitled “Microscopic Feature Dimension Measurement System”, which issued to Fiekowsky on Mar. 25, 2003, teach methods for accurately measuring a line width on a photomask using an optical microscope setup. However, these methods can only measure line width, i.e. lateral dimensions, and cannot provide accurate height information.
Therefore, a need arises for a non-destructive method that is accurate, easy to use, and relatively inexpensive to measure and monitor patterned substrates. The need is met with the present invention which will be explained in the following detailed description.